Electromagnetics

Airborne Electromagnetic (AEM) data are one of the geophysical data types acquired by the Geophysics Group in the Onshore Energy and Minerals Division. AEM systems work by transmitting an electromagnetic signal from a system attached to a plane or helicopter. The signal induces eddy currents in the ground that are detected by receiver coils that are towed below and behind the aircraft, in a device called a 'bird'. Depending on the system used and the subsurface conditions, AEM is able to detect variations in the conductivity of the ground to a depth of around 300m.

Applications of AEM

AEM surveys are more expensive than other airborne geophysical systems and government organisations have only acquired AEM data in relatively small and isolated patches across Australia. AEM surveys also require more complex processing to allow interpretation, and are therefore usually designed to detect particular subsurface targets that are based on a perceived conductivity contrast, for example:

  • The spatial extent of geological features, such as a clay-rich unit in a sedimentary sequence or a graphitic unit in a metamorphic complex
  • The depth of an unconformity between sedimentary cover and the underlying basement rock, or
  • The location of groundwater resources, such as fresh or saline aquifers.

AEM acquisition for the Onshore Energy Security Program

Several new AEM surveys are being acquired as part of the Onshore Energy Security Program (OESP) at Geoscience Australia. The surveys are being designed to reveal new information about regions that are considered prospective for energy resources. Through the innovative approach of acquiring the AEM data at wide line spacings (1- 6km), the surveys will cover relatively large areas. Mineral exploration companies will benefit through an improved understanding of the regional geology, but are also able to collaborate by paying for more detailed data acquisition over their area of interest. More information on AEM acquisition for the OESP is available on the OESP home page.

AEM acquisition for Natural Resource Management (NRM)

Geoscience Australia has also been involved in the acquisition and processing of AEM surveys for NRM, in particular those targeting salinity or groundwater issues in the Murray-Darling Basin. These surveys, such as the Lower Macquarie survey in Central West NSW, were acquired and processed by Geoscience Australia as part of large multi-agency projects managed by the Bureau of Rural Sciences. The surveys have revealed important new information relevant to NRM, such as the distribution of freshwater aquifers in the sediments of the Murray Basin.

Controlled Source Electromagnetics

Controlled Source ElectroMagnetics or CSEM is used in oil and gas exploration and is a remote electromagnetic method that attempts to map the distribution of resistors in a conductive medium. The conductive medium is a combination of the seawater and saltwater filled sedimentary rocks and the resistors are the hydro-carbon filled sediments. A marine CSEM survey is carried out by placing Electro-Magnetics (EM) receivers on the seafloor and then towing a large horizontal electric dipole source over the receivers at 20 to 40m above the seafloor. The changes in the EM field recorded by the EM receivers are then processed to give an estimate of the distribution of resistive rock units within the survey area.

CSEM is typically used late in the exploration cycle to test petroleum drilling targets identified by seismic data. Exploration companies use CSEM to try to determine if these targets are resistive, if they are resistive there is a higher chance that the drilling target may contain either oil or gas.

 

Topic contact: energy@ga.gov.au Last updated: June 4, 2012